This invention relates to a system for promoting removal of noxious components from the exhaust gas of an internal combustion engine which is equipped with a carburetor and in its exhaust line a catalytic converter.
With respect to an internal combustion engine, it is one of the fundamental requisites to success in removing, or at least reducing for the most part, noxious components from the exhaust gas that the air-fuel ratio of a combustible mixture fed to the engine is maintained at a predetermined value with high precision. This requisite is critical when removal of the noxious components is accomplished by catalytic conversion in the exhaust system of the engine.
There has been proposed an excellent catalyst which comprises a plurality of platinum group metals and catalyzes both the oxidation of carbon monoxide and unburned hydrocarbons and the reduction of oxides of nitrogen. This catalyst exhibits its full ability in the exhaust gas from a conventional gasoline engine only when the engine is run with an air/fuel mixture prepared at approximately, if not exactly, the stoichiometric mixing ratio. When the air-fuel ratio of the mixture exceeds the stoichiometric ratio (about 14.8 by weight for air/gasoline mixture), a sharp drop occurs in the conversion efficiency of removing oxides of nitrogen. On the other hand, the efficiency of oxidizing carbon monoxide and unburned hydrocarbons drops sharply when the air-fuel ratio is lowered from the stoichiometric ratio. It is necessary, therefore, to maintain the air/fuel ratio of the combustible mixture at the stoichiometric ratio with accuracy of better than .+-.1%. It was impossible, however, with conventional carburetors to accomplish such a precise control of the air/fuel ratio since the air/fuel ratio depends on physical properties such as density and viscosity of air and fuel which are variables depending on the atmospheric pressure, ambient temperature, and fuel temperature.
In connection with control of the air/fuel ratio, it is known that an actual air/fuel ratio in the running engine can be estimated by measuring the concentration of a certain component of the exhaust gas by the use of an electrical sensor. Useful sensors are known for almost every of major components of the exhaust gas such as oxygen, carbon monoxide, carbon dioxide, hydrocarbons and oxides of nitrogen. For example, an oxygen sensor of the concentration cell type having an ion-conducting solid electrolyte is particularly suitable for detecting slight deviations of the air/fuel ratio from the stoichiometric ratio because the relationship between the output voltage of this sensor exposed to the exhaust gas and the air/fuel ratio of the combustible mixture fed to the engine exhibits a very sharp and great change at the stoichiometric air/fuel ratio.
With respect to an internal combustion engine which is equipped with a carburetor having an air bleed passage opening into a fuel discharge passage and, in the exhaust system, a catalytic converter containing therein a catalyst which catalyzes oxidation of carbon monoxide and hydrocarbons and reduction of oxides of nitrogen, it is an object of the present invention to provide a system for promoting removal of noxious components from the exhaust gas, which system controls the air/fuel ratio of the combustible mixture fed to the engine to the stoichiometric ratio with high precision based on the concentration of a particular component of the exhaust gas measured in the exhaust system at a location upstream of the catalytic converter.